Here's a list of "cheap" ideas I had that I think also have strong story potential. Apologies if any of them have already been posted, I'm new here.
Ill Gotten Gains
A group of well-armed but wounded raiders arrive, each hauling random items they stole from another colony. The group rests for awhile, then attempts to traverse the map and exit (shooting at any colonist they encounter, but not pursuing them). If the colonists defeat them (same programming as for any raid), the player is immediately offered the option to return the goods to the original colony (resulting in a goodwill boost, possibly proportional to the percentage of goods recovered) or to keep the goods for themselves.
Stampede!
An enormous herd of muffalo (20+) spawn on one side of the map and rapidly move to the opposite side (preferably on a route that takes them near colony structures).
Optional: the muffalo may detour to consume player crops.
Optional: the muffalo may attack anyone (or any structures) that get within a certain distance of them.
Most Dangerous Game
Attacking a muffalo could, rarely (<1% of shots), trigger the entire herd to attack the perpetrator. I can imagine stories about running the length of the map pursued by berserk muffalo would be highly entertaining, and it would make hunting require more care from the player. Players could always shoot at the muffalo with other colonists while the original hunter ran, so even if the muffalo didn't give up the chase for several minutes, the hunter wouldn't be doomed.
My Friend Muffalo
a muffalo (or any other animal) takes a liking to a colonist and follows him or her around exclusively. The muffalo attacks anything that attacks that attacks the colonist and is within a few tiles (to avoid the muffalo running to its death in a long range shootout). If the colonist is disabled, the muffalo picks him up and carries him to his bed. Having your last colonist revived by his loyal animal companion would be heartwarming.
Optional: doors open for the muffalo as if it were a colonist (to avoid awkward pathfinding, but at the price of some realism). OR doors do NOT open for the animal. When the character is disabled, the muffalo knocks down all doors needed to get the colonist to his bed.
Squirrel of Caerbannog
Initially, there is no notification that this (rare) event is occurring. Instead, an astute player might note a particular squirrel (or other animal) is eating dead bodies that have been left out. When the squirrel has eaten a certain number of bodies down to 0 hp, a notification pops up informing the player that a local squirrel has developed a taste for human flesh. The squirrel attacks colonists like existing mad squirrels, but moves faster, does more damage, and is much harder to hit than normal. This event would be more realistic if it were implemented with a predatory animal, though the squirrel version is pretty funny.
Little Shop of Horrors
A carnivorous plant spawns. It gradually spreads (using the existing algorithms for grass or other plants). Bullets can reduce the plant to 1 hp, making it inactive until it heals, but can't kill it. This makes fire the only effective way to remove it. It attacks any colonists, animals, visitors, or raiders that come close enough. Killing the occasional boomrat would hopefully give players a chance to eliminate them even if they have no molotov cocktails. Though, if players make no attempt to eradicate them, having most of the world be covered in man-eating jungle would make a great story (as would an attempt to burn the world to finally remove them). A crafty player could also try baiting raiders into a patch.
Optional: colonists could haul a disabled plant to a plant pot before it heals. Potted carnivorous plants would still attack anything in range, but would not spread. If the pot is destroyed, the plant reverts to wild form at 1 hp. I imagine an excited player hauling a defeated plant into a pot in his base, only to find the plant attacks his colonists when it heals, would be pretty funny, and honestly what he should have expected.
